tree 991f23c31af6fceff050210d6aff603a54dfd584
parent f3a1f9324097311cafcd7893066e3e40fe172fd0
author Anders Hartvoll Ruud <andruud@chromium.org> 1537880878 -0700
committer Blink WPT Bot <blink-w3c-test-autoroller@chromium.org> 1537881559 -0400

[css-properties-values-api] Substitution behavior.

Currently, registered properties substitute into other values exactly
like specified (like unregistered properties). This means that, for
a <length>-registered property with a specified value "10em" (for
instance), when that property is substituted via var()-reference,
the tokens "10em" are inserted. This is not correct, and produces the
wrong result with e.g. inherited values.

This CL changes that, by implementing "absolutization" of registered
custom properties: a process which calculates the computed value of
the property, and then produces a token stream usable for substitution
which is equivalent to the computed value.

 * Currently we resolve var()-references on all custom properties
   before applying high-priority properties. This is no longer
   possible, because the true value of a custom property (also
   unregistered) can not be known until the font has been updated.
   Consider: --reg-len: 1em; --unreg:var(--reg-len). Here, the
   computed value of --unreg should be "16px" (assuming a font-size
   of 16px), and not "1em". If we destructively resolve --unreg
   before the font size is known, we end up with the wrong tokens.

   Hence the resolution process has been moved to after the font
   has been updated.

 * Custom properties must also be usable from high-priority properties.
   Since they are no longer resolved beforehand, they are resolved
   "on the fly", non-destructively. "Non-destructively" means that
   resolved token streams are not stored on ComputedStyle, such that
   any var()-references are kept for the _real_ resolution pass after
   the font has been updated.

   This works, because the high-priority properties do not require
   proper "absolute substitution" to produce the correct value.

   Note that font-size is special, in that font-relative units may
   not be used if they arrive via a registered custom property.

 * There is a special resolving pass for registered custom properties
   (ComputeRegisteredVariables). This pass produces non-absolute CSSValues
   for calculation of animation.

 * Parsing of registered properties now happens entirely in
   CSSVariableResolver, and no longer in variable.cc. Having this in
   multiple places is just confusing.

R=futhark@chromium.org

Bug: 641877
Change-Id: Ic705d0808ffcea0ae5db02fb20870767175bb706
Reviewed-on: https://chromium-review.googlesource.com/1240274
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593902}